EMCCD detectors having a photoactive area and a gain register are known in the field. These are a special type of charge-coupled device (CCD) detector; “EM” standing for “electron multiplying”.
In conventional CCD detectors, each photon releases one electron when striking a photoactive layer of the CCD detector; the electron then being trapped in one of a plurality of so-called “potential wells”, where it remains until read-out. During the actual read-out operation, the potential wells are controlled by application of voltages in such a way that the electrons are transferred from their current potential well to the respective next potential well in a “bucket-brigade” fashion until they finally reach a digitization stage. In this process, however, the read-out noise is so high—at least about 20 electrons—that single electrons, and thus single photons, cannot be distinguished from the noise or detected individually. Therefore, conventional CCD detectors cannot be used for standard confocal microscopes.
Like conventional CCD detectors, EMCCD detectors have a photoactive area and, in addition thereto, a gain register to form an amplifier path between the shift register and the output amplifier. Amplification in the amplifier path is accomplished by electron multiplication in a similar manner as in an avalanche diode. Electrons are accelerated by a high voltage to such a degree that they are able to release other electrons by ionization collisions and thus to initiate an electron avalanche. In other words, EMCCD detectors have special potential wells in the form of gain registers, in which the charge transfer in the bucket brigade takes place at such a speed and force that further electrons are generated by electron collision. The further electrons are now themselves transported and accelerated, and release electrons as well. However, the gain factor is quite small, so that a great number of gain stages—usually about 400—need to be passed through to obtain an appreciable gain. When there is sufficient gain, the primary electron finally initiates such an electron avalanche that the number of secondary electrons exceeds the usual read-out noise, thus eventually allowing detection of single photons. In this respect, the EMCCD detectors are similar to the photomultipliers normally used in confocal microscopy, but, for example in the case of the back-illuminated type, they have three times as high a quantum efficiency than the best photomultipliers.
However, the conventional EMCCD detectors are problematic in that their read-out rate is not sufficient for spectral confocal microscopy. Therefore, it has not been possible so far to use EMCCD detectors in applications of spectral confocal microscopy.